CMMG Bolt Hold Open Actuator

[shadow65]

I received my BHOA today.
Install was easy. CMMG added a set screw adjustment to cover a variety of lower specs. Mine took one turn.
The new mag springs and followers that work with these have a stonger spring in them. It will take some rounds through it to seat the spring. CMMG is recommending loading 15-20 rounds until the mag is broke in.
I ran 20 mags through the rifle and it locked back every time.
If you seat the mag sharply, the bolt can slam forward, but then so can a 5.56.
If you hit the butt on the ground it can also come forward.
Hand cycling will only lock the bolt back if pulled sharply. Slow cycling will not consistantly lock the bolt back.

It's a very nice addition to the system. Easy to install, worked on all my lowers, and also works with the conversions.
It's billet so it should last a lifetime.
Dave

[supersix4]

are they shipping the pre-ordered units now?

[austinN4]

Easy to install, worked on all my lowers, and also works with the conversions.
Thanks for posting. Do you know if it will also work with BDM mags?

[shadow65]

John tells me they have 1000 shipping out tomorrow.
The BHOA will work with the BDM mags. You have to buy the CMMG spring and follower made for the BHOA. There is a very slight modification that has to be made to the mag. Instructions are included. It involves a very slight bit of material filed/cut/however you like to do it, out of the rear of the feed lip area of the mag to allow the new follower to work properly. Very simple.
The BHOA mags from CMMG do not need any modification.

[rman43]

Thanks for the report. Hopefully, I will be getting mine soon. I loaned a TacSol upper to CMMG for testing. The BHOA would not work with it. I understand that the upper wouldn't close on the lower with the BHOA installed, but I'm not sure why. At any rate, they told me if I ever bought one of their uppers they would send me one free. I recently ordered and received an M4 LE upper and they wrote on the invoice that they would be sending a BHOA when they ship. I'll probably still have to buy a new magazine or a kit for one of my old CMMG or BDM mags. I'm anxious to try it out - I haven't had a chance to even shoot the upper yet.

Tom

[shadow65]

Tom, they should ship out today. I spoke with John yesterday and they had 1000 ready to ship. I have a proto type that is MIM. The ones sold are billet.
So far as the mag, if you already have mags, I would just get the spring/follower kit. It requires you to file a small(1/16") angle at the back of the feed lip area. Took me about 1 minute.
I hope I can get one of the billets soon. But the guys waiting on them deserve to get there's first.

I would try the BHOA without adjusting the set screw first.
Keep in mind, when new you have to snap the charging handle back to simulate the actual recoil a round would produce.
I was slowly pulling it back and the bolt would not lock every time.
When fired, it locked back 100%
Enjoy.:D

[rman43]

Thanks for the tips - I really appreciate it.

Tom

[shadow65]

I adapted a BDM mag to work with the BHOA. Mine had a very tight fit which was interferring with the operation of the BHOA. I had to file some material off the back of the feed lip tower and the first 3/4" on the back of the mag body. Also, on the inside, back of the mag, the last about 1" of the channel that the follower sits in, has a slight ridge on that needs to be smoothed.
You can see it close to the top by running the follower up with the mag taken apart.
I finished up with 220 sand paper.
You also have to file a 45 degree angle on the back, inside top of the feed tower. Again, this allows the tail of the follower to pop up and actuate the BHOA.
For me, I would stick with CMMG mags but I already have several BDM's. I'll switch a couple over but it can be a pain to get the proper fit to function.

A tip I found today. My follower was dragging in the CMMG mag. If you have to much resistance, the follower will not move the BHOA.
There are two pins in the follower that hold the spring for the top. Mine had a pin not flush. I used a punch and tapped it in flush. No more drag and the follwer worked fine. Before I did this, my BHOA was having problems.
Dave

[rman43]

Dave,

That's good information. CMMG is shipping my BHOA and a new stainless bolt to replace my parkerized one Monday. I still can't believe they offered to do that for free! I have a bunch of BDM mags, but only 2 CMMGs. I may leave the BDMs alone and use them in the TacSols and Nordic and buy adapters for the CMMGs and pick up a few more already set up for the BHOA.

Tom

[shadow65]

It looks like some guys are having issues with the BDM mags. The older mags seem to have slight differences in the build and tolerances.
Some work with a minimum of mods, some are taking more, and some won't work at all.
It's hard to make something to fit universally. I have some older BDM mags. I worked with one, but I hit the feed lips with my Dremel. Now the rounds pop out sometimes under the pressure of the new, stronger spring.
I'll just use my CMMG mags for now. Even the older ones are easy to mod.
Here are some of what has been found to work.
Fromm CMMG:

First things first, the BHOA and follower assembly is a conversion to a conversion. We are going way beyond what the kit or mag was ever meant to do.

I also had to file some material off the front and back of the feed lip tower.

Regarding the BDM mag, it sounds as if the clearance between the back of the BDM mag and magwell is a little snug. See if you can confirm this. If so, the mag may need a small amount of material removed where the actuator drops down. If this is where the tightness is coming from, the actuator will not work since the mag spring will not be able to get it to overcome the binding.
http://www.hunt101.com/data/500/magtip1.jpg

Hitting the mag to hard will cause the bolt to close, this is normal. We did everything we could to mimic the 5.56 but adding the actuator left a slight amount of play which made hard mag insertion result in the bolt closing.

If the bolt is still 1/4" open that can mean one of two things, either the adjustment screw is too high or the radius at the back/center of the bolt itself is contacting the actuator. This radius seems to be pretty arbitrary and seems to vary a lot from kit to kit(all makes we have seen, ours included)
http://www.hunt101.com/data/500/magtip3.jpg


It is best to just install the BHOA with the adjustment all the way down. See if this helps. I will post a diagram tomorrow on the latter issue and the fix.

I would definitely recommend running 3 mags with 15 rounds thru the kit first without the BHOA. This allows the mag to settle in. Hand cycling the kit is not a good indication of how it will run with live ammo. Once you have shot your three, install the BHOA and blast away. You should be GTG.

Edited to add––The BHOA is beveled top and bottom and will self center when the kit closes. Some rifles have tight uppers or tight lower, loose uppers or loose lowers or some combination of all the above. With some set ups, the BHOA will fall out when the gun is opened, with others it will be stuck in either the top or bottom. This is all normal. It is all dependent on the tolerances of the upper and lowers.

One other issue with some BDM mags is there is a little shelf on the inside of the mag on the rear that can impede the follower enough to cause problems activating the actuator.

http://www.hunt101.com/data/500/magtip2.jpg

[rman43]

Thanks for the info shadow! For the time being I'll stick with the CMMGs, but I may get adventuresome later. I have a lot more BDMs on hand.

[SICARIO]

Has anyone tried modifying the BHOA to work with a Tacsol Upper. I'm thinking of buying one to see if it's possible, but figured I'd ask here first.

[rman43]

Has anyone tried modifying the BHOA to work with a Tacsol Upper. I'm thinking of buying one to see if it's possible, but figured I'd ask here first.
I sent a TacSol M4 upper to CMMG for testing with the BHOA. They said it will not work.

[SICARIO]

I sent a TacSol M4 upper to CMMG for testing with the BHOA. They said it will not work.

Hmm. Do you know if they tried any modifications to the BHOA, or did they just try dropping it in?

[rman43]

Hmm. Do you know if they tried any modifications to the BHOA, or did they just try dropping it in?
All they said was that the upper would not close on the lower with the BHOA in place. I'd have to guess that they did not try any modifications. You might give CMMG a shout and get the definitive answer from them. They are very good people to work with.

[azlester]

Ok,maybe I am just having a bad day understanding...or maybe I am just stupid...:haha:
I have a Stainless conversion I purchased in October of '09 along with 3 CMMG 26rd mags. To use the last round bolt hold open i need to purchase new spring/followers and the bho actuator? Correct?
Is there "kit" that includes both items?
In checking around the net there doesn't seem to be too many places stocking the actuator or spring/followers(making it difficult to impossible to get any kind of discount off suggested retail). Same thing for the drop in charging handle. What's up with that? :confused:
As always thank you for your inputs and opinions

[SICARIO]

CMMG has all of the accessories you're looking for on their website: http://cmmginc.secure-mall.com/category/.22-Evolution-187/rec/10

Midway also has the parts (though I don't know if their prices are any better than CMMG:

3 pack spring/follower: http://www.midwayusa.com/viewproduct/?productnumber=264289

BHO Adapter: http://www.midwayusa.com/viewproduct/?productnumber=866746

Charging handle: http://www.midwayusa.com/viewproduct/?productnumber=190957

[Deaj]

The documentation on CMMG's site regarding this adapter two bolt modifications are mentioned and illustrated for older kits apparently manufactured before this adapter was completed. I'd like to get clarification regarding the necessity of these bolt mods. Are the required to make the earlier kit bolts compatible or are these mods meant as steps to be taken if one's older kit has trouble functioning with the adapter?

Thanks!

[HeavyDuty]

I'm using the CMMG BHO with a dedicated Spike's bolt (not a conversion) - no bolt mods were needed. I'd wait and see if you are having problems first.

[shadow65]

Correct. The mods are only if you are having an issue. My older kit did not need the modifications.